Stereotactic localization refers to the localization of an object in a three-dimensional workspace by means of two or more two-dimensional sensors viewing the object along different sight lines. A three-dimensional coordinate system is established in the workspace and the three-dimensional coordinates of the object are calculated from the two-dimensional views obtained by the sensors. Stereotactic localization is important, for example, during invasive surgery when it is necessary to locate in the operating space, with a high degree of accuracy, part of a surgical instrument. For example, the location of the exposed handle of a probe in the operating space may be determined, from which the location of the tip of the probe inside the patient's body may be calculated. In such cases, the location of the tip may be registered in a displayed computerized image of the patient, such as an X-ray, sonogram, or CAT scan.
The sensors used in stereotactic localization may be cameras that sense light emitted by objects to be detected in the workspace. Devices utilizing video cameras as sensors are described in L. Adams, et al., IEEE Computer Graphics and Application Vol. 10 (1990), No. 3, pp. 43-51; M. P. Helibrun, et al., Stereotactic and Functional Neurosurgery 1992, No. 58, pp. 94-98; N. Meitland et al., Proceedings of the 5.sup.th British Machine Vision Conference, York, BMVA Press, 1994 pp. 609-618 and in U.S. Pat. Nos. 5,603,318 and 5,792,147. Alternatively, the objects of interest may be modified to emit another form of energy detected by the sensors. The type of energy used may be radio-frequency radiation (as disclosed in U.S. Pat. No. 5,251,635), sound waves (U.S. Pat. No. 4,012,588), or pulsed DC magnetic fields (U.S. Pat. No. 5,558,091). Devices based on the detection of pulsed infrared light radiation by video cameras are disclosed in U.S. Pat. Nos. 5,622,170, 5,197,476 and 5,792,147.
An array of two-dimensional sensors to be used in stereotactic localizing systems must be calibrated prior to use in order to establish the relative orientation of the sensors in the array and other parameters necessary for locating objects in space. In one calibration procedure, a plurality of target objects are placed in the workspace at points of known three-dimensional coordinates, and stereotactic views of the targets are obtained by the sensors. The resulting calibration data are then used to generate a localization function determining the three-dimensional coordinates of an object in the workspace from the two-dimensional stereotactic views of it obtained by the sensors. Computation of a localization function is described, for example, in Jain, R. et al., Machine Vision, McGraw-Hill, New York 1995. The localization function involves 14 parameters that are calculated from the calibration data. Six of these parameters characterize sensor positions and orientation, two characterize the sensor projections of the focal plane, four characterize optical distortion of the lens, and two characterize scaling factors and aspect ratio. The accuracy of the calculated camera parameters, and hence of the localization function, is limited by the number of target points used for obtaining the calibration data. For the localization function to be accurate in the entire workspace, calibration data must be obtained over a set of target points spanning as large a volume in the workspace as possible and being as dense as possible in that volume.
U.S. Pat. No. 5,603,318 discloses a device consisting of a small number of targets attached to the patient that serve for calibration of sensors in a medical stereotactic procedure. Since the number of targets used for the calibration is small, and is necessarily located outside the patient's body, the calibration data yielded are accurate only near the targets. The accuracy rapidly diminishes to intolerable levels at locations away from the targets including locations of medical interest (e.g. inside a patient's body). This calibration method necessitates that it be performed in the workspace and that the calibration device be present in the workspace during the medical procedure.
Alternative methods in the art use a fixed planar or box-type three-dimensional calibrating target that provides at least three surfaces as calibrating targets. These devices also do not have a sufficiently high target density to provide accurate calibration data for high accuracy localization during a medical procedure.